Antibody against purified (Na ion plus K ion)-ATPase of Electrophorus electric organ has been used to localize the active sodium transport system in brain of a smaller closely-related species at the ultrastructural level. The indirect peroxidase technique was employed. Localization to plasma membranes of neurons and glia was observed. In oligadendroglia the distribution extends to myelinated axons where it is restricted to the outermost edge of compacted myelin. Along axolemma of myelinated axons it is restricted to the portions at the node of Ranvier not covered by myelin.